There is a need to remove and/or recover contaminants from industrial process gas streams. Separation by adsorption is generally preferred over catalytic and reactive methods since reaction byproducts can be avoided and regeneration/reuse of the adsorbent is possible. There is a certain class of contaminants that are very weakly adsorbed in zeolites containing alkali and alkaline earth cations, but the adsorption of such contaminants can be greatly enhanced by exchange with Cu, Ag or Au. This enhancement arises when the physisorption forces are significantly augmented by chemisorption through π-complexation. Molecules of such a class are small enough to enter the zeolite structure and possess an electron structure that easily forms π-bonds with the cations in the zeolite, e.g. carbon monoxide (CO), ethylene (C2H4), propylene (C3H6), etc.
The equilibrium adsorption capacity of CO in AgX and AgY was measured by Huang (J. Catal., v32, 482-491, 1974) as pure component isotherms. About 1.0 mmol/g CO is adsorbed on AgX powder at an equilibrium pressure of 0.1 torr and 25° C. Similar amounts of N2 are adsorbed at pressures above 100 torr. While this CO equilibrium capacity is attractive, these data do not confirm the utility of such materials in real processes designed for removal and/or recovery of CO or other contaminants of interest. More particularly, the working capacity of CO reflecting coadsorption of competitive components in the gas stream and the adsorption kinetics in agglomerated zeolites must be considered. The mass transfer front characteristics and the presence of adsorption inhibitors such as oxidation and reduction agents are also important considerations in practical applications.
One example of these real process effects is the removal of CO from air at normal ambient temperatures. CO and H2 are present in atmospheric air at concentrations of approximately 0.5 ppm to 10.0 ppm. H2 may act as a reducing agent to Ag+, while O2 (about 21 vol %) acts as an oxidizing agent and is also coadsorbed along with N2. Thus, H2 and O2 can be deactivating agents with respect to the Ag+ cation, while O2 and N2 compete for adsorption space with the CO. In addition, at low concentrations of CO, the CO mass transfer front is diffuse and spreads over the entire adsorbent bed, i.e. with no apparent equilibrium zone. For these reasons, the equilibrium CO capacity data obtained from pure component isotherms is of limited value in predicting the effectiveness of an adsorbent for CO removal from an actual process stream.
In addition, large scale adsorbent processing introduces variables and problems absent from laboratory or bench-scale processing such that the production of large quantities of adsorbent with desirable properties is not guaranteed. Some of the problems inherent in processing large quantities of solid materials include uniformly mixing and heating the solid, the distribution of purge flow, purge/solid contact, rapid elimination of water vapor and disposal of waste solutions. Other issues include the costs of dry contaminant-free purge gas, exchange solutions and deionized wash water and the capital equipment required to handle large quantities of solids. Moreover, prior art techniques demonstrate high variability and lack of consistency in performance of such materials.
Given the numerous types of aluminosilicate zeolites combined with variations in Si/Al ratio and charge-balancing cations, much literature on zeolites has been developed. The primary purpose of such technical literature is to demonstrate the synthesis of the material and to characterize its physical, chemical and adsorptive/catalytic properties. Conditions and practices typical in such teachings are the use of gram quantities of adsorbent processed (often in powder form), use of ion exchange solution(s) and deionized wash water in large excess relative to the quantity of adsorbent and minimal requirement for disposal of waste chemicals. Very slow heating is typically applied in combination with large amounts of dry inert purge or evacuation during thermal activation of the material. Such practices are neither economical nor practical for industrial scale production of the adsorbent or catalyst.
The prior art recognizes a number of applications for Ag-exchanged zeolites, e.g. scavenging of H2 from vacuum spaces, Ar/O2/N2 separations, as a bactericide in water purification and separation of liquid phase aromatic hydrocarbon isomers.
Matsch, et al. (U.S. Pat. No. 3,108,706) describes AgX zeolite for use as a H2 getter in maintaining the vacuum insulated annulus space of cryogenic storage containers. In this application, H2 is removed by chemical reaction with the Ag-exchanged zeolite, i.e. the Ag+ and/or excess Ag2O contained in the zeolite are reduced by the H2.
Rosback (U.S. Pat. No. 3,969,276) provides a method of manufacturing an adsorbent comprising ion exchanging either an X or Y zeolite with cations from Group IA, IIA and IB of the Periodic Table. The preferred adsorbent is BaX or BaKX for separating liquid phase mixtures of hydrocarbons.
Chiang, et al. (U.S. Pat. No. 6,432,170) describe a LiAgX zeolite with a silver exchange level from 20% to 70% for separation of Ar/O2 gas mixtures. The aim of this invention is to produce an adsorbent with enhanced Ar selectivity over O2 at a reduced cost (by minimizing the silver content).
A method for preparing AgA zeolite with adsorptive selectivity of N2 over O2 in air and Ar over O2 is disclosed in U.S. Pat. No. 6,572,838 to Sebastian, et al. A single step ion exchange process using an aqueous solution of any silver salt is said to create an adsorbent insensitive to hydroxylation.
U.S. Pat. No. 4,019,880 to Rabo et al. discloses Ag-exchanged zeolites with a high affinity for CO in the presence of water vapor and CO2. Zeolites preferred by this invention include the high silica zeolites of the ZSM-series having 20≦SiO2/Al2O3≦200. Although tolerant to H2O and CO2, these zeolites are subject to deactivation by H2 as the Ag+ cations are reduced by H2.
Coe, et al. (U.S. Pat. No. 4,544,378) relates to a CaX zeolite for air separation, attempting to avoid the damaging effects of both zeolite framework and cation hydrolysis through special attention to the thermal activation step in processing of the adsorbent. LiAgX zeolites are disclosed for N2 adsorption in air separation in U.S. Pat. No. 6,780,806 by Yang et al. Preferred formulations include Si/Al=1.0 with Ag<20% of exchangeable cations. The increased N2 adsorption capacity is attributed to the supplementary weak chemical bonds formed between N2 and Ag clusters due to π-complexation. It is suggested, however, that fully exchanged AgX is not favorable for adsorptive separation due to strong adsorption of N2 at low pressure, resulting in difficulty in desorbing the N2 for regeneration of the adsorbent.
Ag-exchanged zeolites created for applications such as gettering of H2 (U.S. Pat. No. 3,108,706) are thermally treated to reduce, react and/or activate the Ag, resulting in the destruction of part or most of the micropore volume of the zeolite. Thus, catalytic or chemical reactive functions are promoted at the expense of adsorption capacity.
With few exceptions, the prior art implicitly assumes that the adsorption characteristics resulting from bench-scale processing of ion exchanged zeolites prepared for gas separations are automatically reproducible in large-scale manufacture. Such thinking completely ignores the physical and economic limitations in processing industrial-scale quantities of zeolites. Ion exchange, drying and thermal activation steps, each involving heat and mass transfer, do not scale linearly from the gram quantities of adsorbents typical of lab-processing to the large kg quantities required for efficient manufacture. As mentioned above, prior art techniques demonstrate high variability and lack consistency in performance of such materials.
In addition, the high cost of Ag used in the ion exchange solutions, the disposal of their waste and the costs of providing dry purge gas cannot be ignored in large-scale manufacture.
Commonly owned PCT international publication No. WO 03/101587 A1, entitled “Production of High Purity and Ultra-High Purity Gas”, discloses the use of Ag-exchanged zeolites for removing CO in air separation plant prepurifiers. The entire contents of PCT international publication No. WO 03/101587 are incorporated herein by reference.
While the prior art provides several examples of Ag-exchanged zeolites for a variety of separations, the prior art lacks high performance adsorbents exhibiting consistently high dynamic capacity for gas molecules receptive to π-complexation such as CO, C2H4, C3H6 and the like.
It would thus be desirable to consistently provide adsorbents that achieve high working capacity as well as perform well in the presence of high concentrations of gases that would normally be strongly coadsorbed by physisorption (N2, O2, CO2, etc.) or in the presence of strong reducing (e.g. H2) and oxidizing (e.g, O2) agents.